1. Field of the Invention
This invention relates to a high speed yoke driver for the magnetic deflection yoke of a stroke-write display system, and more particularly, to a circuit for driving the deflection yoke of one orthogonal channel of a CRT in which a signal, derived from the integration of the yoke drive current by the yoke inductance, is combined with the vector slope signal component, and is then impressed directly on the deflection yoke thereby enhancing the effective bandwidth of the driving channel.
2. Description of the Prior Art
In stroke-write systems utilizing a CRT (cathode ray tube), the desired information such as alphanumeric characters, is displayed on the CRT faceplate by a series of rapid straight-line strokes of the electron beam similar to the method by which a pen or pencil generates characters on a piece of paper. A stroke vector has a pair of orthogonal components which are proportional to the magnet flux field generated by the horizontal and vertical deflection coils. To display alphanumeric characters, these magnetic flux fields must change at high rates thereby requiring corresponding high speed electrical signals to be applied to the magnetic deflection yokes.
A limitation of the high speed capability of prior art stroke-write systems has been the bandpass characteristic of the channel through which the yoke deflection current is derived. In the generation of alphanumeric characters on the faceplate of a CRT, it is known that the generation of good quality alphanumeric characters requires an effective channel bandwidth which is capable of passing approximately the 7th to 8th harmonic of the electrical signal wave driving the magnetic deflection system. Acceptable alphanumeric characters can be generated if the effective channel bandwidth passes signals through the 5th harmonic of the deflection signal waveform but the sharp corners of the characters tend to be rounded causing each individual character to be somewhat harder to identify. Where the effective channel bandwidth only has the capability of passing the 3rd harmonic of the deflection signal waveform, the characters on the CRT faceplate are significantly rounded and it becomes difficult to identify specific characters. Accordingly, in order to increase the high speed capability of the stroke-write system while still maintaining an acceptable level of character reproduction on the CRT faceplate, it is essential that the effective bandwidth of a driver circuit for the horizontal and vertical deflection yokes being increased to the point where they effectively reproduce the 7th to 8th harmonic of the slope vector component.
In some prior art stroke-write display systems, character information is stored in digital form until gated into the deflection channel for generation of the specific character. Separate storage for the horizontal and vertical signal may be provided and this stored information is periodically presented to the respective channels as required. Since the driver circuits for a magnetic deflection yoke are analog devices, the digital signal representing the vector slope is fed to a D/A converter where an analog equivalent of the stored digital information is derived. Deflection yokes require large current to generate the requisite amount of magnetic flux necessary for beam deflection, and one technique for a current generation has involved the use of a voltage follower power amplifier with negative feedback for DC stability. The analog signal representing character slope is typically a high frequency signal, i.e. above 4 megahertz and is integrated and combined with a low frequency, i.e. below 4 megahertz, to make the composite sweep signal which is then applied to the power amplifier. The output signal from the power amplifier is fed to the respective deflection yoke for deflection of the electron beam in either the horizontal or vertical direction.
U.S. Pat. No. 4,001,806 issued Jan. 4, 1977 to C. Sweeting, assigned to the same assignee, discloses a constant speed stroke-write vector display system of the type heretofore described. As illustrated, in FIG. 1, analog signal representing character slope information is fed to an integration circuit to generate a signal waveform, which when combined with positional information in a summing circuit, is suitable for driving a deflection amplifier. The most significant limitation of this circuit in the production of high speed high quality alphanumeric characters is the effective bandwidth of the integrator and deflection amplifier.
Of general interest are U.S. Pat. No. 3,757,155 issued Sept. 4, 1973 to the present inventor and U.S. Pat. No. 3,515,933 issued June 2, 1970 to J. P. Meyer. Both of these patents describe a circuit configuration which provide some compensation for the high and low frequency characteristics of a deflection yoke thereby improving the overall linearity of the closed loop deflection amplifier.